Differential Global Positioning System (D-GPS) systems enhance the capability of GPS receivers to provide much-improved accuracy from meters to centimeters. A ground-based reference station is involved in a D-GPS system to broadcast the pseudorange difference between the location indicated by GPS satellite signal processing and the known fixed location of the reference station. A GPS receiver may then use the broadcast data to correct its pseudorange by the same amount.
The positioning accuracy of a GPS system is affected by various factors. One such factor is that the receive antenna should, ideally, receive only the direct path GPS signal and filter out all undesired signals, most of which are contributed by ground reflected interference.
A D-GPS system generally requires better suppression of back/side lobes of both right hand circular polarization (RHCP) and left hand circular polarization (LHCP) gain patterns. In order to address this issue, reference antennas have been developed in which radiated antenna elements are sparsely-arranged. In one approach, non-fed antenna elements, which are not connected to a feed circuit, are inserted between two active elements, which are connected to the feed circuit, to improve the antenna performance. In another approach, a factor is used to adjust the spacing between radiated antenna elements to further improve antenna performance.